Detergent compositions in tablet form are known in the art, as discussed below, and some products are now on the market. Tablets have several advantages over powdered products: they do not require measuring and are thus easier to handle and dispense into the washload, and they are more compact, hence facilitating more economical storage.
Detergent tablets are described, for example, in GB 911 204 (Unilever), U.S. Pat. No. 3,953,350 (Kao), JP 60 015 500A (Lion), JP 60 135 497A (Lion) and JP 60 135 498A (Lion); and are sold commercially in Spain.
Detergent tablets are generally made by compressing or compacting a detergent powder. It has proved difficult, however, to strike a balance between tablet strength and ability to disperse and dissolve in the wash liquor. Tablets formed using only a light compaction pressure tend to crumble and disintegrate on handling and packing; while more strongly compacted tablets may be sufficiently cohesive but will then fail to disintegrate or disperse to an adequate extent in the wash.
This problem has proved especially acute with tablets formed by compressing conventionally produced spray-dried powders containing detergent-active compounds and built with insoluble sodium aluminosilicate (zeolite). As the tablet is wetted, highly viscous gel phases are apparently formed which retard or prevent penetration of water into the interior of the tablet.
It would appear that the problem of disintegration in the wash liquor arises to a much lesser extent when sodium tripolyphosphate is present in the formulation, because the ready solubility and high heat of hydration of the phosphate cause it to behave as a tablet disintegrant. Preparation of satisfactory tablets from modern formulations where sodium tripolyphosphate has been replaced by an insoluble material, crystalline sodium aluminosilicate (zeolite), is proving considerably more difficult.
GB 983 243 and GB 989 683 (Colgate-Palmolive) disclose detergent tablets having improved dissolution properties, prepared by compacting spray-dried detergent powders that have been sprayed with water or with aqueous sodium silicate solution in order to reduce the proportion of fine particles (smaller than 100 mesh (US), equivalent to 149 .mu.m) present. Compaction of powders having particle size ranges of 8-100 mesh and 6-60 mesh (US), equivalent respectively to 149-2380 .mu.m and 250-3360 .mu.m, is disclosed. The whole tablet is coated with a film-forming polymer to aid resistance to abrasion and accidental damage. The powders contain high levels of sodium tripolyphosphate.
EP 466 484A (Unilever PLC) published 15 Jan. 1992 discloses detergent tablets of compacted particles having a narrow size cut, and uniformity and regularity of particle shape; benefits are improved disintegration in the wash and attractive appearance.
It has now been found that greatly improved disintegration and dispersion properties may also be obtained from a tablet consisting essentially of a matrix of compacted granules having a wider particle size range than that disclosed in EP 466 484A (Unilever) published 15 Jan. 1992 provided that at least the particles of detergent-active compound and detergent builder are coated with binder/disintegrant before tablet compaction. The benefits are especially apparent in tablets prepared from zeolite-built detergent powders, and from high-bulk-density detergent powders.